1. Field of the Invention
The present invention relates generally to fuel cells. More particularly, it concerns improving the performance of biological fuel cells by reducing the pH gradient between the cathode and anode compartments.
2. Description of Related Art
Biological fuel cells (BFCs) are widely researched today as a means to produce combustionless electrical energy from a wide variety of organic compounds present in water. BFCs can contain either an anode and cathode, in addition to a biological component. The biological component can either be an enzyme or a full cell (e.g., bacterium, microbe, algae, etc.). Chemical fuel cells consume hydrogen (H2) and simple organic compounds (e.g., methane and methanol) by using precious-metal catalysts, usually platinum, and relatively high temperatures. BFCs use microorganism and/or enzymes as the catalysts and are capable of oxidizing many complex organic compounds, as well as simple organic molecules, present in water (Moon et al. 2006, Torres et al. 2007, Topcagic et al. 2006, Bullen et al. 2006). This capability of BFCs opens up the possibility of producing electrical energy directly from biomass feed stocks that are renewable and carbon-neutral fuels (Rabaey et al. 2005, Davis et al. 2007, Rittmann 2008).
All fuel cells have certain common features: (1) an electron donor (the fuel) is oxidized at the anode, which is a conductive solid that accepts the electrons from the donor; (2) a catalyst is needed to carry out the oxidation at the anode; (3) the electrons move through an electrical circuit from the anode to the cathode, which is another conductive solid; (4) at the cathode, the electrons are added to an electron acceptor, usually oxygen (O2); and (5) either protons (H+) move separately from the anode to the cathode or hydroxide ions (OH−) move from the cathode to the anode to maintain electroneutrality in the anode compartment. Failure to transfer the H+ ions from the anode compartment or OH− ions to the anode compartment can result in acidification of the anode compartment and a pH gradient between the compartments.
In chemical proton exchange membrane fuel cells (PEMFCs), an acidic pH condition (i.e., high concentration of H+ ions and a low pH) can facilitate the transport of protons that is required between anode and cathode, as described by the following reactions:Anodic PEMFC reaction: ½H2→H++e−Cathodic PEMFC reaction: ¼O2+H++e−→½H2OHowever, the use of microbes or other biological catalysts in the anodic compartment of a BFC normally requires a near-neutral pH (Bullen et al. 2006, Torres et al. 2008). The low proton concentration in the anodic compartment (0.1 μM at pH 7) contrasts with the relatively high concentration of other ionic components of biological media (buffers, salts in mM range), which are often needed to maintain the operation of BFC biological components. These high concentrations result in a limitation of proton transport between the anode and cathode compartments. To maintain electroneutrality, other ions are transported between the compartments (Rozendal et al. 2006, Chae et al. 2008). The result is a pH gradient, especially at high current densities, in which the anode compartment pH decreases and the cathode compartment pH increases (Gil et al. 2003). The practical effect of the pH gradient is a drop in voltage efficiency, which consequently decreases power generation. In microbial fuel cells, this pH difference was calculated by Rozendal et al. (2007) to be more than 4.4 pH units, which resulted in a potential loss of more than 0.26 V, or approximately 20% loss in available energy. Thus, there is a need for techniques for reducing the pH gradient between the cathode and anode compartments in biological fuel cells.